This invention is directed to an improved method for making small particle zinc oxide sols having utility in diverse applications such as cosmetic/UV sunscreens, UV absorption applications in plastics, paint and glass, general catalysis applications, etc.
As is well known and reported in the literature, zinc oxide has the property of being transparent to visible light but opaque to ultra violet (UV) light, thereby making it very useful in UV screening applications where transparency to visible light is desired.
The opacity of a suspension of fine material is dependent upon the particle size of the material, the difference in the refractive index of the dispersed material and the dispersing medium, the degree of absorption of the light by the particles of material and the wavelength of light used in the opacity measurement.
The refractive index (measure of speed of light in the given substance relative to the speed of light in air) of zinc oxide is relatively high, being.about.2.0, making it a good opacifying pigment, e.g. for paints. Since the refractive index is a fixed characteristic, the approach to alter opacity is to manipulate particle size.
The surface of a zinc oxide crystal effectively reflects visible light. Opacity to visible light reaches a maximum at a particle size of 0.25 micron (250 nm). Smaller particles actually transmit more light than their larger counterparts. This is because particles smaller than.about.0.25 micron no longer simply reflect back the light, but instead, they scatter it in all directions. Some of this scattered light goes forward, becoming transmitted light.
Zinc oxide owes its opacity below 370 nm to its absorbing power, practically no light being reflected. Because absorption is the mechanism of action, it is possible to make transparent to visible light (non-reflecting) zinc oxide formulations that effectively attenuate UV radiation. Particle sizes on the order of.about.0.1 micron or less will provide UV absorption while being effectively transparent to visible light. Above this size, "whitening" will occur.
While a search of the patent literature is not necessary in order to demonstrate the efficacy of transparent zinc oxide coatings on a substrate to block transmission of UV radiation, attention is invited to a brief description of the following patents which are thought to be relevant to the instant invention to be described hereinafter.
U.S. Pat. No. 5,252,356 issued to Yoshida et al. discloses a method of producing a transparent zinc oxide film having an even thickness and which is especially useful as a ultraviolet rays cutoff, comprising applying an organic solution of a zinc salt of a 3-7 carbon fatty acid and a chelate compound of a 5-8 carbon diketone to a substrate and then baking. Preferably, the solution further contains an aluminum, indium, tin, or titanium organometallic compound to provide a zinc oxide film having a higher transparency.
U.S. Pat. No. 5,688,439 issued to Chopin et al. discloses colloidal particulates comprising a core of cerium oxide at least partially coated with a sheathing layer of titanium oxide are well suited for the photocatalytic coating of a variety of substrates to impart anti-UV, anti-reflecting and/or anti-staining properties to the substrate. In Col. 1, line 55 through Co. 2, line 18 four alternative methods of effecting precipitation of titanium oxide onto the cerium oxide are disclosed.
The colloidal dispersions comprise colloids in which the particle size thereof ranges from 5-100 nm, preferably 5-30 nm. The colloidal dispersions are said typically to contain a cerium oxide:titanium oxide ratio by weight of 30/70 to 70/30, preferably 50/50.
Advantageously (co. 3, lines 50-55), the colloidal dispersions additionally contain at least one metal serving as a doping agent to reinforce the photocatalytic activity of the titanium dioxide, e.g. palladium, tungsten, platinum or rhodium.
The present invention is also directed to providing a transparent UV absorber consisting of a layer of one oxide over a "core" of another oxide sol. The problem Applicant has confronted in attempting this approach is the difficulty in making very small particle size transparent sols (less than 50 nm) exhibiting optimum screening of UV radiation by prior art methods known to him.
Stated simply, this problem is the task to which to which the present invention is directed.